October 2018 - Haskell-Cafe

Re: [Haskell-cafe] Impossible types [was [Haskell-prime]: A question about run-time errors when class members are undefined]
by Anthony Clayden 15 Oct '18

15 Oct '18

Transferring from https://mail.haskell.org/pipermail/haskell-prime/2018-October/004396.html Second instalment. On Sat, 13 Oct 2018 at 4:04 AM, <camarao(a)dcc.ufmg.br> wrote: > ... > Ambiguity should be a property of an expression, defined as > follows: if a constraint set C on the constrained type C∪D⇒t of > an expression has unreachable variables, then satisfiability is > tested for C (a definition of reachable and unreachable type > variables is at the end of this message), and: > > - if there is a single unifying substitution of C with the set > of instances in the current context (module), then C can be > removed (overloading is resolved) and C∪D⇒t 'improved' to > D⇒t; > > - otherwise there is a type error: ambiguity if there are two > or more unifying substitutions of C with the set of instances > in the current context (module), and unsatisfiability > otherwise. > > 2. Allow instances to be imported (all instances are assumed to be > exported): > > import M (instance A τ₁ ⋯ τₙ , … ) > > specifies that the instance of τ₁ ⋯ τₙ for class A is > imported from M, in the module where the import clause > occurs. > > In case of no explicit importation, all instances remain > imported, as currently in Haskell (in order that well-typed > programs remain well-typed). > Consider some typical Read instances from the Haskell Prelude > instance Read a => Read [a] where ... > instance Read a => Read (Maybe a) where ... Suppose I have `show . read` where the 'piggy in the middle' type is unreachable (by your definition below); but I want it to be taken as `(Maybe [Int])`. Then I arrange my imports of instances to make visible only ... what? `(Maybe a)` ? Then how do I read the `[a]`? The only instance in scope is `(Maybe a)`. `(Maybe [Int])` ? Then how do I decompose the structure to call the overloads for `read` of the list and of the `Int`? Instance `(Maybe a)` is not in scope, neither is `[a]` nor `Int`. And what if in the same module/same scope I want `show . read` where 'piggy in the middle' is `[Maybe Int]` or `(Maybe [Bool])` ? Then instead of `Read`, I could create a whole swag of specialised classes/methods: ReadMaybeListInt, ReadListMaybeInt, ReadMaybeListBool, ... ? But those could just be regular functions with an explicit signature. Putting an explicit type annotation on each expression seems a whole lot more convenient than mucking around with specialised classes or scope of instances. Before you answer, consider this very common pattern. It's so common there's a base library operator for it: type-level `==` https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.12.0.0…; but the technique dates back to 2004/the HList paper at least: > class TypeEq a a' (b :: Bool) | a a' -> b > instance TypeEq a a True > instance (b ~ False) => TypeEq a a' b Then an example usage > instance (TypeEq e e' b, TypeIf b This That) => instance Elem e (Cons e' tail) where ... Rubric: `Elem` is searching for type `e` in some type-level heterogeneous list; if found (i.e. `TypeEq` returns `True`), the `TypeIf` despatches to `This`, else to `That`. It's crucial for `TypeEq` that both instances are in scope in all places. Ah, but I see the `TypeEq` example does not contain any "unreachable" tyvars by your definition below. In which case your specification is silent as to how type improvement is to behave. (But you claim your approach is instead of FunDeps; then what is to happen with both instances being able to unify with the type of some expression?) Then do you want to change the spec? This was the experience with the github rounds: the spec kept changing; the examples we'd worked through in earlier rounds then didn't work with the new spec. Therefore the claims were impossible to verify with any certainty. > > (Definition: [Reachable and unreachable type variables in constraints] > Consider a constrainted type C∪D⇒τ. A variable a ∈ tv(C) is reachable > from V = tv(τ) if a ∈ V or if a ∈ tv(π) for some π ∈ C such that there > exists b ∈ tv(π) such that b is reachable from V; otherwise it is > unreachable. > For example, in (F a b, X a) ⇒ b, type variable 'a' is reachable from > { b }, because 'a' occurs in constraint F a b, and b is reachable. > Similarly, if C = (F a b, G b c, X c), then c is reachable from {a}.) > > AntC

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[ANNOUNCE] GHC 8.4.4 released
by Ben Gamari 14 Oct '18

14 Oct '18

Hello everyone, The GHC team is pleased to announce the availability of GHC 8.4.4, a patch-level release in the 8.4 series. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.4.4 This release fixes several bugs present in 8.4.3 These include, - A bug which could result in memory unsafety with certain uses of `touch#` has been resolved. (#14346) - A compiler panic triggered by some GADT record updates has been fixed (#15499) - The `text` library has been updated, fixing several serious bugs in the version shipped with GHC 8.4.3 (see `text` issues #227, #221, and #197. - A serious code generation bug in the LLVM code generation, potentially resulting in incorrect evaluation of floating point expressions, has been fixed (#14251) As always, the full release notes can be found in the users guide, https://downloads.haskell.org/~ghc/8.4.4/docs/html/users_guide/8.4.4-notes.… Thanks to everyone who has contributed to developing, documenting, and testing this release! As always, let us know if you encounter trouble. How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: https://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page https://www.haskell.org/ghc/ GHC developers' home page https://ghc.haskell.org/trac/ghc/ Haskell home page https://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: https://ghc.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: https://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: https://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at https://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Some GHC developers hang out on #haskell on IRC, too: https://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here:

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ANN: DBFunctor – Functional Data Management / ETL Data Processing in Haskell
by Nikos Karagiannidis 14 Oct '18

14 Oct '18

Dear all, I am pleased to announce the release on Hackage of a new Haskell ETL Library, namely DBFunctor-0.1.0.0. (“ETL” stands for Extract Transform and Load and is the standard technology for accomplishing data management tasks in Data Warehouses / Data Marts and in general for data migrations, or for preparing data for any analytic purpose (Ad hoc queries, data exploration/data analysis, Reporting and Business Intelligence, feeding Machine Learning algorithms, etc.)) Version 0.1.0.0 can be used for *in-memory data processing* without the need of some external database. It is just a Haskell library linked in your Haskell program that enables full SQL functionality over tabular data (e.g., CSV files) but also the ability to write a full ETL data processing flow. Key features of this release are: 1. *Julius DSL*: A Type-Level Embedded Domain Specific Language (EDSL) for ETL 2. Supports all known relational algebra operations 3. Implements the ETL Mapping and other typical ETL constructs and operations 4. Applicable to all kinds of tabular data 5. In-memory, database-less data processing For more information please check out the project homepage: https://github.com/nkarag/haskell-DBFunctor Also there is a tutorial that can help you to easily get started with Julius DSL: https://github.com/nkarag/haskell-DBFunctor/blob/master/doc/JULIUS-TUTORIAL… * Learn how to implement your ETL flows with Julius DSL * Learn how to implement common ETL operations (append a delta to some target table, or create a surrogate key column etc.) * Build your own aggregate or analytic functions (string_agg, or a montlhy running total) * Express complex queries with Julius (equivalent to the WITH-clause in SQL) * and many more ... For any issues/problems with the DBFunctor package please open an issue on github. Happy data processing! Thank you. Best Regards, Nikos

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Creating the network-bsd package
by Kazu Yamamoto 12 Oct '18

12 Oct '18

Hi, We the network package maintainers are planning to release network v3.0.0.0 which enables extending socket addresses. In this version, we will remove the Network.BSD module which version 2.7 already deprecates. Unfortunately, it appeared that many users use Network.BSD and there is no good alternative. So, I would like to split it into another package, say, network-bsd. Some years ago, the same thing happened to Network.URI, resulting in the network-uri and network-uri-flag packages. Currently, I'm planing to take the same approach: ---- flag network-bsd library -- ... if flag(network-bsd) build-depends: network-bsd >= 3.0, network >= 3.0 else build-depends: network-bsd < 3.0, network < 3.0 --- Or ---- build-depends: network >= 2.5 && < 2.7, network-bsd >= 2.5 && < 2.7, network-bsd-flag == 0.1.* --- Question: is this a correct appoarch? Or are there any better ways to split Netwrok.BSD? --Kazu

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NYC Haskell Developer Position
by Christopher Lewis 11 Oct '18

11 Oct '18

Karamaan Group is an investment firm based in Manhattan. We are looking for an outstanding software developer to develop tools for financial analysis and knowledge management. Since we are investors and not traders, our focus is not on building trading systems, but on business and data analysis. We strongly value people who take pride in their work and have a craftsman’s dedication to building bespoke products. Our ideal candidate is an experienced software developer with a penchant for abstraction and analysis, a solid sense of organization, and an ability to communicate and persuade. While we are looking for a candidate who demonstrates an intense focus on quality, she also needs a master engineer’s intuition for how to make trade offs that are a necessary part of day-to-day software development. Candidates should have some formal training in a quantitative field and a keen interest in building robust and elegant software. This is a high-impact, high-visibility position where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influence on our workflow. The ideal candidate will have production experience with Haskell or another functional language, relational databases and relational data modeling, Python, and Java. Additionally, as this is a startup environment, the ideal candidate will be comfortable filling many different technology roles, and contributing to the success of the firm wherever they can, not just in agreement with their job description. Karamaan Group is an investment adviser and manager catering to family offices and institutional investors. We value innovative thinking, encourage an entrepreneurial atmosphere, and enjoy spirited discussions. Please send your CV and cover letter to recruitment at karamaan dot com. Please do not reply to the original poster.

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What is the best way to search information on Haskell Cafe?
by Rodrigo Stevaux 09 Oct '18

09 Oct '18

I am new to mailing lists. Like really new.

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Re: [Haskell-cafe] Well typed OS
by Barak A. Pearlmutter 08 Oct '18

08 Oct '18

At a high level, you seem to be proposing encoding and enforcing safety properties of executables using a type system. One instantiation of this idea is proof-carrying-code (Necula and Lee, 1996, see https://en.wikipedia.org/wiki/Proof-carrying_code) which ensures safety properties of machine code. The spin-off Cedilla Systems built a mobile OS (intended for phones) around this, with the idea that third-party apps would run unprotected with safety guaranteed solely by sophisticated use of types. The Fox Project, http://www.cs.cmu.edu/~fox/, was philosophically similar, and their web page contains a bunch of interesting pointers to relevant work. For whatever reason, this stuff never got much penetration. But in today's Internet of Things, with malware being injected into mobile phones and cars and light bulbs and toothbrushes and smart dust, it may deserve another look.

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Looking for maintainers for all my packages
by John Goerzen 06 Oct '18

06 Oct '18

Hi folks, I would like to hand over my remaining Haskell packages to good hands. Please reply, CCing me, if you would like to volunteer. Thanks! These packages are: Libraries: ConfigFile HSH MissingH MissingPy (looks like MattBrown uploaded it a bit in 2012) anydbm ftphs hslogger magic testpack Programs: datapacker gtkrsync hpodder twidge txt2rtf Obsolete, no maintainer needed: darcs-buildpackage dfsbuild gopherbot hg-buildpackage srcinst - John

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[Call for Contributions] Haskell Communities and Activities Report, November 2018 edition (35th edition)
by Mihai Maruseac 06 Oct '18

06 Oct '18

Dear all, We would like to collect contributions for the 35th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 4 November 2018 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac FAQ: Q: What format should I write in? A: Any format is ok, I will transpose the submission to the format in use at the time of publication. Previous editions have used a LaTeX format, with the template that is available at: http://haskell.org/communities/11-2018/template.tex And the associated LaTeX style file at http://haskell.org/communities/11-2018/hcar.sty You can use those to edit and preview your entry, but it is very likely that by the time of publication the entire HCAR pipeline would be changed to a more modern one. You will receive a copy of the draft, prior to publication, to ensure that everything looks ok. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. If using LaTeX, you can use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then. PNG is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Which pretty print library?
by waldmann 04 Oct '18

04 Oct '18

Dear Cafe, what is today's answer to this old question: https://stackoverflow.com/questions/9761507/which-pretty-print-library In particular, I agree with author's: "The thing that worries me about these [many pretty-printing libraries] is that many of them have many releases. This implies they've had features added, bugs fixed, etc. But have they all had the same bugs fixed?" Meanwhile, there's https://hackage.haskell.org/package/prettyprinter "This package solves all these issues, and then some." (clearly an instance of https://xkcd.com/927/ ) GHC is using pretty-1.1.3.6 ? - J.W.

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